Barely two months after rolling out two-factor authentication, Twitter has beefed up its login procedures yet again, both to improve security and to make two-factor available to more Twitter users worldwide.

Twitter launched two-factor authentication in late May with a system based on SMS messaging. While that was good enough for many users, however, it did present some problems.

For one thing, verification via SMS is only available via supported mobile carriers, which isn't all of them. For another, using SMS as a security mechanism relies on the SMS delivery channel being secure, and some carriers' text messaging systems might not be.

On Tuesday, Twitter rolled out an update to its two-factor authentication scheme that can optionally make use of the Twitter mobile app for Android and iOS, rather than SMS.

"Simply tap a button on your phone, and you're good to go," Twitter security engineer Alex Smolen wrote in a blog post. "This means you don't have to wait for a text message and then type in the code each time you sign in on twitter.com."

The app approach offers other advantages, as well. Because it's based on public-key cryptography, it's inherently more secure than the SMS approach. The mobile app generates a public/private key pair and Twitter only stores the public key, while the private key never leaves the user's phone. As a result, an attacker won't be able to fake a login even if Twitter's authentication server is compromised.

When the app receives a login verification request, it will also tell you details about the browser that is being used to make the login attempt, including its approximate location – so if you see any suspicious logins from Iran, you'll know not to approve them.

The app also generates a "backup code" during the setup process, which it advises you to write down and keep in a safe place. In the event you ever lose your phone, you can use the backup code to login to Twitter, un-enroll your old phone from login verification, and enroll your new one.

As before, login verification is optional and can be enabled from any Twitter account's Settings panel. To take advantage of the new, app-based authentication process, you'll need to update to version 5.9 of the Twitter app for iOS or version 4.1.4 of Twitter for Android, both of which shipped on Tuesday. ®